For example, as a power supply unit for a charger, a small-sized switching power supply is generally used which comprises an output voltage detecting circuit, a constant current control circuit, and a photocoupler on a secondary side to perform constant voltage control while offering a constant current dropping characteristic.
A switching power supply has also been proposed which allows a further size reduction by omitting the output voltage detecting circuit, the constant current control circuit, and the photocoupler from the secondary side and allowing a primary side to provide these functions (see, for example, Europe Patent Application Publication No. 1211794). This switching power supply performs what is called PFM control by, for a constant voltage region, detecting a voltage generated in an auxiliary winding in a transformer and adjusting an off time of a switching element so that the detected voltage is maintained constant. The switching power supply also offers the constant current dropping characteristic by, for a constant current region, controlling the oscillation frequency of the switching element so as to maintain constant the duty ratio of a secondary current flowing through a secondary winding in the transformer.
However, in the conventional switching power supply, with a fixed load, a more stable switching operation is disadvantageously likely to make the oscillation frequency of the switching element more stable and thereby cause noise (electric noise). Thus, to prevent the possible noise, the conventional switching power supply requires an anti-noise component such as a filter circuit. The anti-noise component prevents a reduction in the size of a power source.